ORGANS OF MOVEMENT IN THE PELECYPODA. (it 



Natatory Lobes. The heteropods or swimming gastropods 

 develop from the ventral portion of their body a vertical fin or 

 swimming disk (Ixxxvi, 99), which, in the male only, is furnished 

 with a Slicker placed on its margin. Carinaria has also a caudal 

 lobe or tail expansion for swimming. 



Opisthobranchiate gastropods are furnished with lateral lobes 

 proceeding from the sides of the anterior portion of the foot, or 

 from the mantle ; in Lobiger ilxxxix, 66), a pair of them is devel- 

 oped upon each side of the body. 



The pteropods (xlii), develop a pair of dorsal swimming- 

 lobes, but the foot is small or obsolete in most of them. In 

 Cleoclora it is combined with the fins ; in Spinalis a rudimentary 

 foot supports an operculum. 



The Columellar Muscle. There is but one attachment of the 

 gastropod to its shell ; namely, by means of the columellar 

 muscle, by which the inner face of the columella is directly 

 united with the posterior portion of the body of the animal. It 

 passes underneath the mantle, greatly thickening the body wall, 

 and terminates upon the inner face of the operculnm, so that by 

 its contractions the operculum and shell are approximated, and 

 the animal withdrawn within the latter. The form of this 

 muscle depends on that of the shell, and in the conical, non- 

 spiral shells especially, varies greatl}^ from its normal develop- 

 ment. Thus it is horseshoe-shaped in Capulus (Ixvi, 30) ; it is 

 divided into two portions, one of which lies on either side of the 

 anterior part of the animal, in Fissurella. In Haliotis the 

 animal is coiled around it, and its insertion, instead of being on 

 the columella, is on the middle of the inner wall of the shell 

 itself, upon which it forms a large oval impression (Ixxxiii, 11). 



In the genus Clausilia an accessor}^ spathuliform piece 

 ( Claiisilium^ iii, 42 j, is united to the columellar axis of the shell 

 hj an elastic pedicle, and appears to act as a purchase in assist- 

 ing the columellar muscle, when the animal desires to withdraw 

 within its shell. 



ORGANS OF MOVEMENT IN THE PELECYPODA. 



The foot (iii, 50, 52), is developed in most bivalve mollnsks 

 and is extremel}^ flexible, having laj^ers of circular muscular 

 fibres for its extension, and longitudinal bands for its retraction. 

 Its degree of development varies with the habits of the genera ; 

 in Pholas, Nucula, etc., it shows a sort of pedal disk, whilst in 

 such genera as Unio, Venus, it is compressed and securiform ; in 

 'Mycetopus it is very long and much dilated at its extremit}^ ; 

 whilst in Lucina it is somewhat cylindrical, and in Mytilus 

 tongue-like (iii, 40, 41). In the attached bivalves, such as 

 Ostrsea, it is either not developed or exists in a rudimentary 

 state subsidiary to the glands which secrete the byssus. 



